Girls slap, right? Guys hit.
But if you were intending to hurt someone and wanted to slap them, you wouldn't go for their back, if only because it normally wouldn't be showing, and he is wearing a top in the picture.
I'm thinking it was some kind of dare, or a game or something.
Hence, it's going to be a guy, not a girl, only guys are dumb enough to do that **** .